Fluids and steam vapor withdrawn from geothermal sources contain dissolved noncondensable gases such as carbon dioxide, hydrogen sulfide and ammonia, which flow with the steam when geothermal liquid and vapor sources are separated. The presence of these gases reduces the net power output of a power plant using steam containing noncondensable gases when compared to a power plant using steam that does not contain noncondensable gases. In addition, some of the gases, such as carbon dioxide and hydrogen sulfide, are corrosive. Furthermore, once the steam has been utilized, i.e., to drive the turbine, the discharge of the vapor or condensate containing H.sub.2 S causes an environmental problem.
To deal with this problem one method is to condense the steam and reboil the condensate in a tube and shell heat exchanging system upstream of the turbine, i.e., prior to introducing the geothermal source steam to the turbine. See U.S. Pat. No. 4,330,307, incorporated herein by reference. A drop in steam temperature, which can be made small, between the two sides of the heat exchanger drives the heat transfer, and about 90 to 95% of the noncondensable gases and a small fraction of steam (about 5%) may be vented and treated separately to remove and dispose of H.sub.2 S gas. The CO.sub.2 can be vented into the atmosphere. By this process approximately 90% of the hydrogen sulfide and other noncondensable gases can be removed, while keeping the steam close to the temperatures and pressures produced at the wellhead in the geothermal field and without requiring any chemical treatment of the main flow stream through the power plant.
Another method is to send the geothermal steam to a direct contact reboiler in which the geothermal steam is used to heat a relatively cool condensate flowing through the reboiler, whereby the noncondensable gases are cooled and separated from the steam. See U.S. Pat. No. 4,534,174, incorporated herein by reference.
A problem, however, still exists in that small amounts of hydrogen sulfide, carbon dioxide, ammonia and perhaps other noncondensables are still present in the so called "clean steam" resulting from processes such as those described in U.S. Pat. No. 4,330,307 and 4,534,174 because the condensate at the bottom of the condensing side of such an upstream reboiler is in a chemical equilibrium in relationship with the feed steam that contains the noncondensables. Due to existing and possibly future stringent, emission standards, even the small amounts of H.sub.2 S present in steam must be controlled in some situations, either for attaining emission standards or for reducing acid gases in turbines and condensers. Removal of this small amount of H.sub.2 S also may be beneficial in reducing chemical and maintenance requirements for H.sub.2 S abatement and simplifying the condenser design. In some situations removal of even small amounts of residual CO.sub.2 may be desirable.
It is therefore an object of the present invention to provide a method for enhancing the removal of noncondensable gases from geothermal steam from which the majority of the noncondensable gases has already been removed by an upstream method, such as, those methods described in U.S. Pat. Nos. 4,330,307 and 4,534,174. This and other objects of the invention will be apparent from the following description, the appended drawing and from practice of the invention.